Electronic flash can be used to take sharp photographs
of events occurring at high speed. Most flash units
designed for amateur photographers can provide flashes
of light as short as 30 millionths of a second.
This is sufficient to "stop" such events as
the impact of a tennis ball with a racket, a foot with
a football, and the burst of a balloon or a popcorn
kernel. The main problem to solve in taking photographs
of these events--or simply in using electronic flash
to view them directly--is to trigger the flash discharge
when the event is occurring.

Three types of triggers and an electronic delay circuit
are described in this section. The simplest trigger,
a metallic switch, closes automatically when the event
occurs. One form of this triggered is described below.
Two other types, sound and photogate, are described
on the pages that follow. All of these triggers have
the function of closing the flash terminals electronically.

Notes about the electronics:
Our use of circuit diagrams assumes that the reader is familiar
with basic electronics. Parts for the circuits are inexpensive
and are available from local electronics stores and catalog
outlets. For parts listings or kits for the various circuits,
see Make your own.

Contact trigger

A contact trigger is simply a metallic switch whose
leads may be connected directly to a flash unit. (They
may also be connected to a delay
circuit for timing synchronization.) When the switch
is closed by, say, dropping a rubber ball on it, a flash
unit will be discharged after the preset delay. By varying
the delay, the collision can be observed in various
stages.

A large area contact trigger that works well for dropped
objects uses 2 20-cm squares of stiff cardboard, 2 20-cm
squares of aluminum foil, and 1 20-cm square of black
construction paper or poster board. Do the following
to construct it. A diagram is shown below.

1) Smooth any wrinkles out of a 20-cm square of Al
foil. Then glue the foil to the same size cardboard.
Tape the bare end of a 1-m wire lead to one corner of
the foil.

2) Smooth out another 20-cm square of foil and glue
it to black construction paper of the same size. Tape
another 1-m wire lead to a corner of this foil.

3) Cut out a 15-cm square hole from the second square
of cardboard, leaving a square frame of 2.5-cm width.
Glue this frame onto the foil prepared in step 2.

4) Arrange the pieces as shown in the diagram. The
cardboard frame acts as a spacer to separate the pieces
of foil. When an object is dropped onto the upper piece,
it is forced into contact with the lower.

Something interesting to try with the contact trigger
is to drop a soft rubber ball or water balloon onto
the trigger. If using a water balloon, protect the trigger
with plastic. Vary the delay to capture the collision
in various stages.